In recent years, wireless terminal apparatuses for mobile communication such as mobile phones have been rapidly developed. When a radio wave arrives at a wireless terminal apparatus from a base station, the radio wave includes multipath waves produced from reflection, scattering, diffraction, or the like due to terrain, structures, and the like in its propagation path, and the radio wave has randomly changing amplitude and phase depending on the location. When receiving a radio wave from the base station as well as moving within its propagation path, fading (i.e., signal level drops, including variation in instantaneous values) occurs due to multipath propagation of the radio wave. As a result, a digital communication environment suffers from increased errors in codes and significant degradation of transmission quality. Thus, if evaluating the communication performance of a wireless terminal apparatus, it is desirable not only to evaluate its static characteristics in an RF anechoic chamber, but also to evaluate its performance in a multipath propagation environment. Hence, the applicant of the present application proposed antenna evaluation apparatuses (also referred to as “spatial multipath waves generating apparatuses” or “fading emulators”) such as those described in Patent Literature 1 and Non-Patent Literatures 1 to 4.
FIG. 14 is a block diagram showing a configuration of a prior art antenna evaluation apparatus described in Patent Literature 1. The antenna evaluation apparatus includes: a plurality of transmitting antennas (referred to as “scatterer antennas” hereinafter) 121-1 to 121-7 disposed at regular intervals on the circumference of a circle with a radius r; an antenna under measurement 122 disposed near a center of the circle, such as a diversity antenna; and a control and measurement apparatus 100 connected thereto. The control and measurement apparatus 100 includes: a network analyzer 111, a divider 112, a phase-shift circuit 113, an attenuation circuit 114, a D/A converter 115, and a computer 110. The network analyzer 111 generates a radio frequency signal, and the divider 112 divides the generated radio frequency signal in accordance with the number of the scatterer antennas 121-1 to 121-7. The phase-shift circuit 113 and the attenuation circuit 114 adjust the phases and amplitudes of the divided radio frequency signals. The adjusted radio frequency signals are radiated from the scatterer antennas 121-1 to 121-7, respectively. Radio-frequency signals received by the antenna under measurement 122 are inputted to the network analyzer 111. The computer 110 controls the network analyzer 111 and controls the amounts of phase adjustments made by the phase-shift circuit 113 and the amounts of amplitude adjustments made by the attenuation circuit 114 through the D/A converter 115. The antenna evaluation apparatus controls the amplitudes and phases of radio frequency signals to be radiated from the scatterer antennas 121-1 to 121-7, and thus controls the property of a multipath propagation environment (a fading environment, etc.) which is formed around the center of the circle by the radiated radio frequency signals. Then, a performance in an actual use environment is evaluated by the antenna under measurement 122 disposed near the center of the circle.
In addition, in recent years, a MIMO (Multi-Input Multi-Output) antenna apparatus for transmitting and receiving radio signals of a plurality of channels using a plurality of antenna elements simultaneously has been developed. FIG. 15 is a schematic diagram showing a MIMO wireless communication system including: a MIMO transmitter 200 with two transmitting antennas 201 and 202; and a MIMO receiver 210 with two receiving antennas 211 and 212. The MIMO transmitter 200 converts a data stream to be transmitted into a plurality of (in this case, two) substreams, and transmits the respective substreams from corresponding transmitting antennas 201 and 202. In this case, a first substream transmitted from the transmitting antenna 201 travels through a first channel 221 and arrives at the receiving antenna 211, and travels through a second channel 222 and arrives at the receiving antenna 212. Likewise, a second substream transmitted from the transmitting antenna 202 travels through a third channel 223 and arrives at the receiving antenna 211, and travels through a fourth channel 224 and arrives at the receiving antenna 212.
Citation List
Patent Literature
PATENT LITERATURE 1: Japanese Patent Laid-open Publication No. 2005-227213.
Non-Patent Literature
NON-PATENT LITERATURE 1: Tsutomu Sakata et al., “Evaluation of Mobile Terminal Antennas using Spatial Fading Emulator”, Matsushita Technical Journal, Vol. 52, No. 5, pp. 70-75, October 2006.
NON-PATENT LITERATURE 2: Tsutomu Sakata et al., “Channel Capacity Measurements of a MIMO Antenna under a Rayleigh-fading Channel by a Spatial Fading Emulator”, Proceedings of the 2007 IEICE Society Conference, B-1-9, September 2007.
NON-PATENT LITERATURE 3: Tsutomu Sakata et al., “A Multipath Measurement System with a Variable Power Angular Spectrum for Handset MIMO Antennas”, IEICE Technical Report, Vol. 108, No. 5, pp. 13-18, April 2008.
NON-PATENT LITERATURE 4: Tsutomu Sakata et al., “An Evaluation of the MIMO Transmission Characteristics in a Cluster Propagation Environment Using a Spatial Fading Emulator”, IEICE Technical Report, Vol. 108, No. 429, pp. 121-126, April 2009.